The present invention is related to a method and apparatus for controlling the power level in the electromagnetic induction (EI) cooker which requires that the direction of current flow in the working coil is alternated by making the power transistor of an inverter turned on-off. The magnetic field generated in the working coil is alternated by the alternating direction of the current flow in the working coil. By controlling the pulse width modulation and the time duty ratio in combination a certain power level can be established by user.
To control the power level in electromagnetic induction (EI) cooker in the prior art, the time duty ratio was control fed to alternate the power level. By controlling the time duty ratio, the power transistor was turned on-off during the settled limited time at the power level to produce maximum output power of EI cooker. A pulse width modulation control method to control the power level by controlling the turn-on time of power transistor with the width of pulse signal during a limited time.
However, when using the time duty ratio control method, low output is available. The power level is controlled easily when the output is low and beat noise caused by the differences in frequencies are not produced in multi-burner type EI cooker having many working coils because the frequency to turn the power transistor turned "on-off" is constant. On the other hand, flickering can occur by changing the load current at the on-off duration of power transistor and the stable operation of EI cooker can not be obtained because of this transicent phenomena.
When using the pulse width modulation control method, power can be obtained continuously and the power level can be kept uniformly. On the other hand, detection of the input current is difficult because the load current is small when the power level is less than 200 W, and the differences in frequencies causes serious beat noise when the power levels are in the respectively different multi-burner type EI cooker.